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my inner geek <geek@ifeden.com> Wrote:
>What are quantum dots?
>Do we currently have storage and retrieval systems using this method?
>How does this compare in efficiency to molecular tape?
I wrote this for the list on Oct 7 of last year.
Unless we find a replacement for field effect transistors Moore's law will eventually come to an end, although IBM's recent technique of using copper instead of aluminum in a IC probably gave us an additional 5 or 10 years breathing room. Power dissipation in a FET can be a real problem, as is trying to interconnect billions of different transistors, even worse, when the FET channel becomes very short quantum effects start to seriously degrade performance. We can't change the laws of physics so we need a way to turn Quantum Mechanics from an enemy to a friend.
Back in 1991 Craig Lent proposed we abandon transistors and capacitors, stop using voltages to encode information and start using individual electrons in a Quantum Cellular Automation (QCA). His idea was to have each cell of the automation be made of 4 quantum dots arranged in a square and have each cell contain 2 extra electrons. The electrons repel each other so they will always try to get into quantum dots as far apart as possible, that is the dots at the diagonal of the square. There are only 2 stable states such a cell can be in:
|X 0 | |0 X | | | | | | STATE 0 | | STATE 1 | | | | | |0___________X| |X___________0|
There is no direct connection between the 4 quantum dots but the 2 extra electrons can move from dot to dot in a cell by quantum tunneling. The electrons can't tunnel to other cells in the automation (a small increase in distance means a huge decrease in the probability of successfully tunneling) but the electrons in each cell can still feel how the charge is distributed in a neighboring cell and will always stay as far apart as possible. For example if you have a long row of such cells and you change the first one to state 0 then that will change the second cell to state 0 and that will change the third cell to state 0 and so on all the way down the line. A signal sent in this way uses no current so almost no heat is generated.
Because a cellular automation interacts with adjacent cells you could intertwine several such "wires" to process information. Lent also designed but did not build a quantum cellular automation adder.
Six years ago not many were very impressed with Lent's idea because they felt the dots could never be made accurately enough and the slightest imperfection would let the 2 electrons leak away in about a nanosecond. In the August 15 1997 issue of Science Lent and his associates report they have actually build one cell of the automation. The cell operated just as Lent said it would, it can be in only 2 states and can stay in one state for many minutes and the electrons do not leak out.
The quantum dots in this prototype are huge, 8 microns across, so he had to keep it at cryogenic temperatures, but Lent says in the article "QCA architecture is scaleable to molecular levels and performance actually improves as the size of the device is decreased. [...] a molecular sized QCA would function at room temperature." Unlike transistors, the smaller these things get the better they work
In an editorial in the same issue of Science Terry Fountain of University College London call this new work "A new and promising possibility to shrink computer architecture [...] by factors as much as 50,000 compared to the smallest feasible transistor. [...] the remaining hurdles though daunting are largely technical".
John K Clark jonkc@att.net
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